Explicitly clean linux-system.axf and xen-system.axf

Explicitly clean linux-system.axf and xen-system.axf

When doing a make clean, only the output image currently configured to
build is being removed.  However, one would expect all build artifacts
to be removed when doing a 'make clean' and when switching between Xen
and Linux builds, it is easy to accidentally run an older build than
intended.  Simply hardcode the axf image file names.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>